Resistance welding a porous metal layer to a metal substrate utilizing an intermediate element

ABSTRACT

One aspect of the present disclosure provides a method of manufacturing an orthopedic prosthesis. This particular method includes providing a porous metal layer ( 22 ) positioned against a metal substrate at an interface between the porous metal layer and the metal substrate. Additional steps include providing an electrode ( 132 ) and providing an intermediate element or coating positioned between and in contact with the electrode and the porous metal layer where the intermediate element includes tantalum in contact with tantalum of the porous metal layer. In a further step, an electrical current is directed to the interface between the porous metal layer and the metal substrate to bond the porous metal layer to the metal substrate.

CLAIM OF PRIORITY

This application is a U.S. National Stage Application under 35 U.S.C. 371 from International Application No. PCT/US2015/037589, filed Jun. 25, 2015, which application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/025,185, filed on July 16, 2014, the benefit of priority of which is claimed hereby, and of which is incorporated by reference herein in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to medical technology and in particular aspects to systems and methods for manufacturing orthopedic prostheses. Illustratively, some aspects of the present disclosure relate to methods for resistance welding a porous metal layer to a metal substrate utilizing an intermediate element.

BACKGROUND OF THE DISCLOSURE

Orthopaedic prostheses are commonly used to replace at least a portion of a patient's joint to restore or increase the use of the joint following traumatic injury or deterioration due to aging, illness, or disease, for example.

To enhance the fixation between an orthopedic prosthesis and a patient's bone, the orthopedic prosthesis may be provided with a porous metal layer. The porous metal layer may define at least a portion of the bone-contacting surface of the prosthesis to encourage bone growth and/or soft tissue growth into the prostheses. The porous metal layer may be coupled to an underlying metal substrate.

SUMMARY

The present disclosure, in some aspects, provides apparatuses and methods for manufacturing orthopedic prostheses. For example, such apparatuses and methods can involve resistance welding a porous metal layer to an underlying metal substrate. The resistance welding process can involve directing an electrical current through the porous layer and the substrate, which dissipates as localized heat to cause softening and/or melting of the materials, especially at points of contact along the interface between the porous layer and the substrate. The softened and/or melted materials undergo metallurgical bonding at the points of contact between the porous layer and the substrate to fixedly secure the porous layer onto the substrate. The present inventors have realized that providing an intermediate element between a porous metal layer and a metal substrate can provide certain resistance welding benefits. Such benefits are not required of all embodiments, but they can include providing a more uniform pressure distribution across the welded surface, a preservation of the porous surface of the porous layer, and control of material transfer (e.g., minimization, elimination, calculated transfer, etc.) to a porous metal of the orthopedic prosthesis, e.g., from an electrode or intermediate member.

In one particular aspect, the present disclosure provides a method of manufacturing an orthopedic prosthesis. One step includes providing a porous metal layer positioned against a metal substrate at an interface between the porous metal layer and the metal substrate. In another step, a coated surface of an electrode is brought into contact with the porous metal layer. In another step, an electrical current is directed to the interface between the porous metal layer and the metal substrate to bond the porous metal layer to the metal substrate.

In another aspect, the present disclosure provides a method of manufacturing an orthopedic prosthesis. One step includes providing a porous metal layer positioned against a metal substrate at an interface between the porous metal layer and the metal substrate. In another step, an electrode is provided. Another step includes providing an intermediate element positioned between and in contact with the electrode and the porous metal layer where the intermediate element includes tantalum in contact with tantalum of the porous metal layer. In another step, an electrical current is directed to the interface between the porous metal layer and the metal substrate to bond the porous metal layer to the metal substrate.

In still another aspect, the present disclosure provides a method of manufacturing an orthopedic prosthesis. One step includes providing a porous metal layer positioned against a metal substrate at an interface between the porous metal layer and the metal substrate. In another step, an electrode is provided. Another step includes providing an intermediate member positioned between and in contact with the electrode and the porous metal layer where the intermediate member is formed separately from the electrode and includes tantalum in contact with tantalum of the porous metal layer. In another step, an electrical current is directed to the interface between the porous metal layer and the metal substrate to bond the porous metal layer to the metal substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document.

FIG. 1 is an elevational view of a prosthetic proximal femoral component, the proximal femoral component including a porous metal layer coupled to an underlying metal substrate;

FIG. 2 is a cross-sectional view of the proximal femoral component of FIG. 1;

FIG. 3 is a front elevational view of an exemplary apparatus used to assemble the proximal femoral component of FIG. 1,

FIG. 4A is a schematic diagram of the apparatus of FIG. 3, the apparatus including fixtures and weld heads that are shown in an open position to receive the proximal femoral component;

FIG. 4B is a schematic diagram similar to FIG. 4A, the fixtures and the weld heads of the apparatus shown in a closed position to hold the porous metal layer against the metal substrate of the proximal femoral component;

FIG. 5 is a graphical depiction of the average bond strength between various porous layers and metal substrates in accordance with Example #1;

FIG. 6 is another graphical depiction of the average bond strength between various porous layers and metal substrates in accordance with Example #2;

FIG. 7 is another graphical depiction of the bond strength between various porous layers and metal substrates in accordance with Example #3;

FIG. 8 is a graphical depiction of the tantalum concentration gradient in a diffusion bonded sample and in a resistance welded sample;

FIG. 9 is a graphical depiction of the titanium concentration gradient in a diffusion bonded sample and in a resistance welded sample;

FIG. 10 is a scanning electron microscope image taken along the interface between a porous component and a substrate of a diffusion bonded sample;

FIG. 11 is a scanning electron microscope image taken along the interface between a porous component and a substrate of a resistance welded sample;

FIG. 12 is another graphical depiction of the bond strength between various porous layers and metal substrates in accordance with Example #6;

FIG. 13 is a scanning electron microscope image taken along the interface between a porous layer and metal substrate in accordance with Example #6;

FIG. 14 is a scanning electron microscope image taken along the interface between a porous layer and metal substrate in accordance with Example #7; and

FIG. 15 is another graphical depiction of the bond strength between various porous layers and metal substrates in accordance with Example #7.

FIG. 16 is a microscopic image of a control sample of a porous metal.

FIG. 17 is a microscopic image of the same type and grade of porous metal as in FIG. 16 except that during a welding step the porous metal was contacted by an electrode having a textured surface.

FIG. 18 is a microscopic image of the same type and grade of porous metal as in FIG. 16 except that during a welding step the porous metal was contacted by an electrode having a non-textured surface.

FIG. 19 is a perspective view of an electrode in accordance with one example of the present disclosure.

FIG. 20 is a schematic diagram of another exemplary welding apparatus according to the present disclosure.

FIG. 21 is a perspective view of an intermediate member in accordance with one aspect of the present disclosure.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate exemplary examples of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, an orthopedic prosthesis is provided in the form of a proximal femoral component 10 (e.g., a hip stem). While the orthopedic prosthesis is described and depicted herein in the form of a proximal femoral component 10, the orthopedic prosthesis may also be in the form of any number of other orthopedic prostheses or implants, or components thereof, including but not limited to a distal femoral component, a distal or proximal tibial component, an acetabular component, or a humeral component.

Proximal femoral component 10 of FIG. 1 includes stem 12 and neck 14, which is configured to receive a modular head (not shown). It is also within the scope of the present disclosure that the head may be integrally coupled to neck 14. In use, with stem 12 of proximal femoral component 10 implanted into the intramedullary canal of a patient's proximal femur, neck 14 and the head (not shown) of proximal femoral component 10 extend medially from the patient's proximal femur to articulate with the patient's natural acetabulum or a prosthetic acetabular component. Stem 12 of proximal femoral component 10 includes an exterior, bone-contacting surface 18 that is configured to contact bone and/or soft tissue of the patient's femur.

As shown in FIG. 2, proximal femoral component 10 includes a metal substrate 20 and a porous metal layer 22 coupled to the underlying substrate 20. Porous layer 22 may be disposed within recess 26 of substrate 20. With porous layer 22 defining at least a portion of bone-contacting surface 18, bone and/or soft tissue of the patient's femur may grow into porous layer 22 over time to enhance the fixation (i.e., osseointegration) between proximal femoral component 10 and the patient's femur.

Substrate 20 of proximal femoral component 10 may comprise a biocompatible metal, such as titanium, a titanium alloy, cobalt chromium, cobalt chromium molybdenum, tantalum, or a tantalum alloy. According to an exemplary example of the present disclosure, substrate 20 comprises a Ti-6Al-4V ELI alloy, such as Tivanium® which is available from Zimmer, Inc., of Warsaw, Ind. Tivanium® is a registered trademark of Zimmer, Inc.

Porous layer 22 of proximal femoral component 10 may comprise a biocompatible metal, such as titanium, a titanium alloy, cobalt chromium, cobalt chromium molybdenum, tantalum, or a tantalum alloy. Porous layer 22 may be in the form of a highly porous biomaterial, which is useful as a bone substitute and as cell and tissue receptive material. It is also within the scope of the present disclosure that porous layer 22 may be in the form of a fiber metal pad or a sintered metal layer, such as a Cancellous-Structured Titanium™ (CSTi™) layer, for example. CSTi™ porous layers are manufactured by Zimmer, Inc., of Warsaw, Ind. Cancellous-Structured Titanium™ and CSTi™ are trademarks of Zimmer, Inc.

A highly porous biomaterial may have a porosity as low as 55%, 65%, or 75% and as high as 80%, 85%, or 90%, or within any range defined between any pair of the foregoing values.

An example of such a material is a highly porous fiber metal pad. Another example of such a material is a CSTi™ layer. Another example of such a material is manufactured using a selective laser sintering (SLS) or other additive manufacturing or rapid prototyping types of processes. Yet another example of such a material is produced using Trabecular Metal™ technology generally available from Zimmer, Inc., of Warsaw, Ind. Trabecular Metal™ is a trademark of Zimmer, Inc. Such a material may be formed from a reticulated vitreous carbon foam substrate which is infiltrated and coated with a biocompatible metal, such as tantalum, by a chemical vapor deposition (“CVD”) process in the manner disclosed in detail in U.S. Pat. No. 5,282,861, the disclosure of which is expressly incorporated herein by reference. In addition to tantalum, other metals such as niobium, or alloys of tantalum and niobium with one another or with other metals may also be used.

Generally, the porous tantalum structure includes a large plurality of ligaments defining open spaces therebetween, with each ligament generally including a carbon core covered by a thin film of metal such as tantalum, for example. The open spaces between the ligaments form a matrix of continuous channels having no dead ends, such that growth of cancellous bone through the porous tantalum structure is uninhibited. The porous tantalum may include up to 75%-85% or more void space therein. Thus, porous tantalum is a lightweight, strong porous structure which is substantially uniform and consistent in composition, and closely resembles the structure of natural cancellous bone, thereby providing a matrix into which cancellous bone may grow to provide fixation of proximal femoral component 10 to the patient's femur.

The porous tantalum structure may be made in a variety of densities in order to selectively tailor the structure for particular applications. In particular, as discussed in the above-incorporated U.S. Pat. No. 5,282,861, the porous tantalum may be fabricated to virtually any desired porosity and pore size, and can thus be matched with the surrounding natural bone in order to provide an optimized matrix for bone ingrowth and mineralization.

When porous layer 22 of proximal femoral component 10 is produced using Trabecular Metal™ technology, as discussed above, a small percentage of substrate 20 may be in direct contact with the ligaments of porous layer 22. For example, approximately 15%, 20%, or 25%, of the surface area of substrate 20 may be in direct contact with the ligaments of porous layer 22.

Referring next to FIG. 3, apparatus 100 is provided for resistance welding porous layer 22 to substrate 20 of proximal femoral component 10. Apparatus 100 is also illustrated schematically in FIGS. 4A and 4B. Apparatus 100 includes housing 110, one or more braces or fixtures 120 a, 120 b, one or more weld heads 130 a, 130 b, within housing 110, each having an electrode 132 a, 132 b, transformer 140, a power source or current generator 150, and controller 160. Each component of apparatus 100 is described further below.

Housing 110 of apparatus 100 defines an internal chamber 112 that is sized to receive at least one prosthesis, such as proximal femoral component 10 of FIGS. 1 and 2. According to an exemplary example of the present disclosure, housing 110 of apparatus 100 creates a vacuum environment or an inert environment in chamber 112 during the resistance welding process. In one particular example, chamber 112 of housing 110 is flushed with an inert gas (e.g., argon) and controlled to have a dew point less than about −60° C. and an oxygen concentration less than about 10 ppm.

Housing 110 may be at least partially transparent to enable a user to see inside chamber 112. Also, housing 110 may include one or more openings 114 to enable a user to access chamber 112. To maintain a vacuum environment or an inert environment in chamber 112, housing 110 may be in the form of a glovebox. In other words, each opening 114 may include a glove (not shown) or another suitable barrier that extends into chamber 112 to receive the user's hand while maintaining a seal around opening 114.

Fixtures 120 a, 120 b, of apparatus 100 contact proximal femoral component 10 to hold proximal femoral component 10 in place within housing 110 of apparatus 100. Fixtures 120 a, 120 b, may be moved apart to an open position (FIG. 4A) to receive proximal femoral component 10, and then fixtures 120 a, 120 b, may be moved together to a closed or clamped position (FIG. 4B) to hold proximal femoral component 10 in place. It is within the scope of the present disclosure that the closed position of fixtures 120 a, 120 b, may be adjustable to enable apparatus 100 to receive and hold prostheses of different shapes and sizes.

Electrodes 132 a, 132 b, on weld heads 130 a, 130 b, of apparatus 100 are connected to transformer 140 and current generator 150 via wires 152 a, 152 b, respectively. As shown in FIG. 4A, each electrode 132 a, 132 b, faces a corresponding side of porous layer 22. More specifically, contact surface 134 a, 134 b, of each electrode 132 a, 132 b, faces a corresponding side of porous layer 22. According to an exemplary example of the present disclosure, contact surface 134 a, 134 b, of each electrode 132 a, 132 b, is designed to substantially match the contour of the corresponding side of porous layer 22. In this example, each electrode 132 a, 132 b, is able to make close, even contact with proximal femoral component 10. Depending on the shape of proximal femoral component 10, the corresponding contact surface 134 a, 134 b, may be concave, convex, or planar, for example. Such weld heads or electrodes can be formed partially or wholly with any suitable conductive material including but not limited to copper, copper alloy, tungsten, tungsten alloy, molybdenum, molybdenum alloy, or any combination of metals or conductors suitable as electrode material known to those skilled in the art.

In some examples, an electrode contact surface such as contact surfaces 134 a, 134 b will be particularly textured, e.g., incorporate one or more surface features or elements, or will otherwise be configured so that structural and/or other characteristics (e.g., surface texture, strut features, surface porosity, pore features, etc.) of a porous metal structure contacted by the electrode during a welding step will be preserved or substantially preserved, when such preservation is desired, after the welding step has been completed. Illustratively, an electrode surface can be equipped with a particular microarchitecture that helps prevent or inhibit structural changes from occurring on or within a porous metal structure during a welding step. With some designs, employing this sort of surface, as compared to using a smooth, even, or generally non-textured surface of the same general size, can reduce the amount of contact between the electrode and the porous metal, and in this regard, it will be understood that the amount of contact between an electrode and a porous metal over a given area can be set at any suitable level. Such texturing, etc. can be incorporated into an electrode surface upon initial formation of the electrode, or an existing electrode surface can be modified to have such features. In this regard, such texturing, etc. can be imparted to an electrode surface, for example, by cutting away, grinding away or otherwise removing material from an initial electrode piece to provide a particular surface texture or other surface microarchitecture, or by welding, adhering or otherwise adding material to an existing electrode piece to provide a particular surface texture or other surface microarchitecture, or by casting or otherwise initially forming an electrode piece to have a particular surface texture or other surface microarchitecture. In some forms, a knurled electrode surface will be utilized. Such texturing, etc. can include any number and type of surface features or elements including but not limited to one or more projections, grooves, ridges, corrugations, peaks, valleys, rings, bands, bumps, bulges, lumps, knobs, protuberances, dimples, depressions, dents, and/or other suitable projections or indentations. Such surface features or elements can be of any suitable size and shape, and they can occur randomly along an electrode surface, or they can be arranged in regular or non-random fashion (e.g., a pattern, grid or array) along an electrode surface or region. In some forms, an electrode surface will be textured or otherwise configured so that it somewhat mimics or approximates one or more surface features of a porous metal structure to be contacted by the electrode during a welding step.

Referring now to FIGS. 16-18, the first of these figures is a microscopic image of a control sample of a porous metal, while FIG. 17 is a microscopic image of the same type and grade of porous metal as in FIG. 16 except that the porous metal was contacted by an electrode having a textured surface (45 degree, 0.040 pitch) during a welding step as described herein. A comparison of FIGS. 16 and 17 shows a substantial preservation of features of the porous metal structure. FIG. 18 is a microscopic image of the same type and grade of porous metal as in FIG. 16 except that the porous metal was contacted by an electrode having a non-textured surface during a welding step as described herein. A comparison of FIGS. 17 and 18 shows a considerable difference in preservation of features of the porous metal structure.

With reference now to FIG. 19, shown is an illustrative electrode 200 according to one aspect of the present disclosure. Electrode 200 includes an electrode contact surface 201 from which a plurality of generally pyramid-shaped projections 202 project. These particular pyramid-like projections, while certainly useful in certain examples of the present disclosure, are merely illustrative of the type, shape, etc. of projections contemplated. Projections and other suitable surface elements or features of any suitable shape, size and configuration can be incorporated into an electrode surface as discussed elsewhere herein.

In addition, an electrode contact surface as disclosed herein, whether or not textured, can incorporate an optional coating, for example, where a coating (e.g., a metallic coating) is formed onto an outer surface of an electrode such as electrode 132 a to provide all or part of the electrode contact surface 134 a. Referring again to FIG. 19, in another illustrative example, electrode 200 can incorporate an optional coating or attached layer 203 that provides all or substantially all of the electrode contact surface 201. In such aspects, it will be understood that layered and non-layered coatings can be formed onto an electrode or other weld head component in any suitable manner for forming a coating on a substrate including but not limited to by thermal spray, plasma spray, electron beam deposition, laser deposition, chemical vapor deposition, dipping, and cold spray techniques. A formed layer or other coating can be continuous or non-continuous across an electrode contact surface. As well, a formed layer or other coating can be porous or essentially non-porous. In certain embodiments, a coating is comprised of one or more identifiable coating layers formed onto an electrode or other weld head component where any one of these layers can have any suitable thickness, for example, in the range of about 0.005 mm to about 0.10 mm, or in the range of about 0.020 mm to about 0.20 mm, or in the range of about 0.050 mm to about 0.30 mm, or in the range of about 0.10 mm to about 0.50 mm, or in the range of about 0.30 mm to about 1.0 mm.

In one illustrative example, an electrode will be formed with a first metallic material such as a copper alloy, and an outer surface of this electrode will be coated with a second metallic material such as a tantalum or a tantalum alloy (e.g., using a CVD process) to provide all or part of an electrode contact surface. Such a coated electrode can then be used in a resistance welding step as disclosed herein, for example, where a porous metal layer (e.g., layer 22) incorporating tantalum is being welded to an underlying metallic substrate (e.g., substrate 20) formed with titanium or a titanium alloy. Such coatings can reduce or eliminate the unwanted transfer of an electrode material (e.g., the copper alloy or first metallic material in the above example) to the porous metal layer during a welding step. Illustratively, the porous metal layer might be formed with tantalum or at least include its own tantalum outer coating that is brought into contact with the tantalum coating on the electrode during a resistance welding step. Illustrative coatings for electrodes can be or include tantalum, niobium, titanium, cobalt chromium, cobalt chromium molybdenum, and combinations and alloys thereof. While not necessary to broader aspects of the disclosure, in some embodiments, a portion of a coating on an electrode or other weld head component will be transferred to the porous metal layer as a result of a resistance welding step, and in some instances, such a transfer will be expected or even desired. In certain methods, the amount of transfer, if any, will be controlled during one or more welding steps.

While in some instances a coating is formed onto an outer surface of an electrode to provide an intermediate element between the electrode and a porous metal layer (e.g., layer 22) to which a metal substrate (e.g., substrate 20) is to be resistance welded, in some other instances, other types of intermediate elements are employed. For example, in some preferred embodiments, an intermediate element includes an intermediate member (e.g., a sheet or layer) that is formed separately from an electrode for subsequently being positioned between the electrode and a porous metal layer (e.g., layer 22) to which a metal substrate (e.g., substrate 20) is to be resistance welded. Such an intermediate member may or may not be adhered or otherwise affixed or coupled to the electrode prior to a resistance welding step. When such members are in a sheet or sheet-like form, the members, or any portion thereof, can have any suitable thickness, for example, in the range of about 0.005 mm to about 0.10 mm, or in the range of about 0.020 mm to about 0.20 mm, or in the range of about 0.050 mm to about 0.30 mm, or in the range of about 0.10 mm to about 0.50 mm, or in the range of about 0.30 mm to about 1.0 mm.

Referring now to FIG. 20, shown are intermediate members 300 a, 300 b being utilized in an apparatus 100′ that is otherwise similar to that shown in FIGS. 3, 4A and 4B. In this regard, intermediate members 300 a, 300 b face corresponding sides of porous layer 22. More specifically, first sides 301 a, 301 b of intermediate members 300 a, 300 b face corresponding sides of porous layer 22. Opposite, second sides 302 a, 302 b of intermediate members face corresponding electrodes 132 a, 132 b. Apparatus 100′ is shown in an open type of position, for example, as might occur during loading of the various pieces. Such an apparatus can then be converted to a closed type of position as discussed elsewhere herein, for example, where various pieces are clamped down or otherwise manipulated so that the intermediate members make contact with the electrodes and the porous layer and/or other surfaces of the orthopedic construct. An electrical current can then be directed to one or more interfaces occurring between the porous layer and the metal substrate using any of the steps, parameters or techniques disclosed herein for bonding the porous layer to the metal substrate.

According to an exemplary example of the present disclosure, first sides 301 a, 301 b are made to substantially match the contour of the corresponding sides of porous layer 22, while second sides 302 a, 302 b are made to substantially match the contour of the corresponding electrodes prior to a welding step, although in other aspects the intermediate members are shaped as part of the resistance welding process. In such examples, each intermediate member is able to make close, even contact with proximal femoral component 10. Depending on the shape of proximal femoral component 10, the intermediate members 300 a, 300 b can include curved and/or planar features, for example, and any suitable number of intermediate members may be utilized. Such intermediate members can be formed to have any suitable size and shape (e.g., a sheet or sheet-like element having any suitable thickness), and they can be formed partially or wholly with any suitable conductive material including but not limited to copper, copper alloy, tungsten, tungsten alloy, molybdenum, molybdenum alloy, or any combination of metals or conductors suitable as electrode material known to those skilled in the art. In addition, intermediate members such as those shown in FIG. 20 can be utilized as disposable elements, for example, being used as few as once or for multiple welding processes.

In some examples, sides or surfaces of intermediate members such as those shown in FIG. 20 will be particularly textured as described elsewhere herein, e.g., incorporating one or more surface features or elements, or will otherwise be configured so that structural and/or other characteristics (e.g., surface texture, strut features, surface porosity, pore features, etc.) of a porous metal structure contacted by such sides or surfaces of the intermediate members during a welding step will be preserved or substantially preserved, when such preservation is desired, after the welding step has been completed. An illustrative example of one such intermediate member 400 is shown in FIG. 21. Member 400 includes a first side 401 from which a plurality of generally pyramid-shaped projections 402 project. These particular pyramid-like projections, while certainly useful in certain examples of the present disclosure, are merely illustrative of the type, shape, etc. of projections contemplated. Projections and other suitable surface elements or features of any suitable shape, size and configuration can be incorporated into an intermediate member as discussed elsewhere herein.

In addition, a side or surface of an intermediate member such as those shown in FIG. 20, whether or not textured, can incorporate an optional coating that is to come into contact with a porous metal layer (e.g., layer 22) during a resistance welding step, for example, where a coating (e.g., a metallic coating) is formed onto a side of an intermediate member such as first sides 301 a, 301 b. Referring again to FIG. 21, in another illustrative example, intermediate member 400 can incorporate an optional formed coating or attached layer 403. In such aspects, it will be understood that layered and non-layered coatings can be formed onto an intermediate member that is formed separately from an electrode in any suitable manner for forming a coating on a substrate including but not limited to by thermal spray, plasma spray, electron beam deposition, laser deposition, chemical vapor deposition, dipping, and cold spray techniques. A formed layer or other coating can be continuous or non-continuous across an intermediate member side or surface. As well, such formed layer(s), coatings, etc. can be porous or essentially non-porous. In certain embodiments, a coating is comprised of one or more identifiable coating layers formed onto an intermediate member that is formed separately from an electrode where any one of these layers can have any suitable thickness, for example, in the range of about 0.005 mm to about 0.10 mm, or in the range of about 0.020 mm to about 0.20 mm, or in the range of about 0.050 mm to about 0.30 mm, or in the range of about 0.10 mm to about 0.50 mm, or in the range of about 0.30 mm to about 1.0 mm.

In one illustrative example, an intermediate member that is formed separately from an electrode will be formed with a first metallic material such as a copper alloy, and a first side or surface of this intermediate member will be coated with a second metallic material such as a tantalum or a tantalum alloy (e.g., using a CVD process), or it will have at least one separately-formed layer of a second metallic material attached thereto for coming into contact with a porous metal layer during a resistance welding step. Such coatings or attached layers can reduce or eliminate the unwanted transfer of an intermediate member material (e.g., the copper alloy or first metallic material in the above example) to the porous metal layer during a welding step. Illustratively, the porous metal layer might be formed with tantalum or at least include its own tantalum outer coating that is brought into contact with the tantalum coating or attached separately-formed layer on the intermediate member during a resistance welding step. Illustrative coatings or attached, separately-formed layers for attachment to intermediate members that are formed separately from an electrode can be or include tantalum, niobium, titanium, cobalt chromium, cobalt chromium molybdenum, and combinations and alloys thereof. While not necessary to broader aspects of the disclosure, in some embodiments, a portion of a coating or attached, separately-formed layer on an intermediate member can be transferred to the porous metal layer as a result of a resistance welding step, and in some instances, such a transfer will be expected or even desired. In certain methods, the amount of transfer, if any, will be controlled during one or more welding steps.

Referring again to FIGS. 4A and 4B, weld heads 130 a, 130 b, of apparatus 100 may be configured to hold porous layer 22 against substrate 20 during the resistance welding process. More particularly, weld heads 130 a, 130 b, may be configured to hold porous layer 22 within recess 26 of substrate 20 during the resistance welding process. Like fixtures 120 a, 120 b, described above, weld heads 130 a, 130 b, may be moved away from proximal femoral component 10 to an open position (FIG. 4A) to receive proximal femoral component 10, and then weld heads 130 a, 130 b, may be moved toward proximal femoral component 10 to a closed or clamped position (FIG. 4B) to hold porous layer 22 within recess 26 of substrate 20. The open and/or closed positions of weld heads 130 a, 130 b, may be controlled using one or more stops 136 a, 136 b, that contact corresponding flanges 138 a, 138 b, on weld heads 130 a, 130 b, to limit movement of electrodes 132 a, 132 b. It is within the scope of the present disclosure that the closed position of each weld head 130 a, 130 b, may be adjustable, such as by moving stops 136 a, 136 b, to enable apparatus 100 to receive and hold prostheses of different shapes and sizes.

Optionally, apparatus 100 may include additional braces or fixtures (not shown) that are configured to hold porous layer 22 against substrate 20 during the resistance welding process. More particularly, these additional fixtures may be configured to hold porous layer 22 within recess 26 of substrate 20 during the resistance welding process.

The pressure used to hold porous layer 22 against substrate 20 of proximal femoral component 10 during the resistance welding process may be sufficiently low to avoid deforming or compressing porous layer 22 while still resisting movement of porous layer 22 relative to substrate 20. Thus, the weld pressure should not exceed the compressive yield strength of substrate 20 or porous layer 22. If the compressive yield strength of porous layer 22 is about 4,000 psi (27.6 MPa), for example, a suitable weld pressure may be as low as 100 psi (0.7 MPa), 500 psi (3.4 MPa), or 1,000 psi (6.9 MPa), and as high as 2,000 psi (13.8 MPa), 2,500 psi (17.2 MPa), or 3,000 psi (20.7 MPa), or within any range defined between any pair of the foregoing values, for example. Porous layer 22 may be provided in a substantially final shape before the resistance welding process to avoid having to compress or otherwise shape porous layer 22 during the resistance welding process. As a result, the thickness of porous layer 22 and the contact area between porous layer 22 and substrate 20 may remain substantially unchanged during the resistance welding process. As discussed above, the weld pressure may be applied by weld heads 130 a, 130 b, when weld heads 130 a, 130 b, are in the closed position (FIG. 4B) and/or by additional fixtures (not shown) of apparatus 100.

Controller 160 of apparatus 100, which may be in the form of a general purpose computer, is coupled to transformer 140 and current generator 150 to control the operation of electrodes 132 a, 132 b. Controller 160 of apparatus 100 may also control the evacuation of housing 110 and/or the flushing of housing 110 with an inert gas (e.g., argon). Additionally, controller 160 of apparatus 100 may control movement of fixtures 120 a, 120 b, and/or weld heads 130 a, 130 b, between their respective open positions (FIG. 4A) and closed positions (FIG. 4B).

In use, proximal femoral component 10 is loaded into housing 110 of apparatus 100. With porous layer 22 properly disposed against substrate 20 of proximal femoral component 10, controller 160 may be operated to move fixtures 120 a, 120 b, and/or weld heads 130 a, 130 b, from their respective open positions (FIG. 4A) toward their respective closed positions (FIG. 4B). The approach pressure (i.e. the pressure at which fixtures 120 a, 120 b, and/or weld heads 130 a, 130 b, approach proximal femoral component 10 before making contact with proximal femoral component 10) may be less than the above-described weld pressure to avoid damaging the components. For example, the approach pressure may be as low as 10 psi (0.07 MPa), 30 psi (0.2 MPa), or 50 psi (0.3 MPa), and as high as 70 psi (0.5 MPa), 90 psi (0.6 MPa), or 110 psi (0.8 MPa), or within any range defined between any pair of the foregoing values.

After proximal femoral component 10 is loaded into housing 110 of apparatus 100, controller 160 may be operated to evacuate chamber 112 of housing 110 and/or to flush chamber 112 of housing 110 with an inert gas (e.g., argon). The vacuum or inert environment within housing 110 of apparatus 100 may substantially prevent proximal femoral component 10 from oxidizing, absorbing atmospheric contaminants, and/or becoming discolored during the resistance welding process.

Controller 160 may then continue moving fixtures 120 a, 120 b, and weld heads 130 a, 130 b, into their respective closed positions (FIG. 4B) to hold both porous layer 22 and substrate 20 of proximal femoral component 10 in place within housing 110. As discussed above, the weld pressure (i.e., the pressure at which fixtures 120 a, 120 b, and/or weld heads 130 a, 130 b, come to hold proximal femoral component 10 during the resistance welding process) may be as low as 100 psi (0.7 MPa), 500 psi (3.4 MPa), or 1,000 psi (6.9 MPa), and as high as 2,000 psi (13.8 MPa), 2,500 psi (17.2 MPa), or 3,000 psi (20.7 MPa), for example.

Next, controller 160 may be operated to initiate current flow from current generator 150 to transformer 140. Current generator 150 may operate at a power of 4 kJ, 6 kJ, 8 kJ, 10 kJ, or more, for example. With a textured or non-textured contact surface 134 a, 134 b, of each electrode 132 a, 132 b, positioned against porous layer 22 of proximal femoral component 10, the weld current flows from one electrode (e.g., electrode 132 a via wire 152 a), through proximal femoral component 10, and out of the other electrode (e.g., electrode 132 b via wire 152 b). In an exemplary example, the source electrode 132 a, 132 b, may deliver a weld current to proximal femoral component 10 as low as 20 kA, 30 kA, or 40 kA, and as high as 50 kA, 60 kA, or 70 kA, or within any range defined between any pair of the foregoing values, for example, to produce weld current densities as low as 25 kA/m² (3.9 kA/cm²), 35 kA/m² (5.4 kA/cm²), or 45 kA/m² (7.0 kA/cm²), and as high as 55 kA/in² (8.5 kA/cm²), 65 kA/in² (10.1 kA/cm²), 75 kA/in² (11.6 kA/cm²), or 85 kA/in² (13.2 kA/cm²), or within any range defined between any pair of the foregoing values. As the weld current flows through proximal femoral component 10, controller 160 may maintain the weld pressure of fixtures 120 a, 120 b, and/or weld heads 130 a, 130 b.

According to Ohm's Law (P=I²*R), the weld current (1) that flows through porous layer 22 and substrate 20 of proximal femoral component 10 dissipates as heat, with the amount of heat generated being proportional to the resistance (R) at any point in the electrical circuit. When different materials are used to construct porous layer 22 and substrate 20, the resistance (R) may be highest at the interface between porous layer 22 and substrate 20. Therefore, a large amount of heat may be generated locally at points of contact between porous layer 22 and substrate 20.

According to an exemplary example of the present disclosure, the heat generated is sufficient to cause softening and/or melting of the materials used to construct porous layer 22 and/or substrate 20 which, in combination with the weld pressure used to hold porous layer 22 against substrate 20, causes surface metallurgical bonding to occur at points of contact between porous layer 22 and substrate 20. It is also within the scope of the present disclosure that metallurgical bonding may occur at points of contact within porous layer 22. For example, if porous layer 22 is in the form of a fiber metal pad, metallurgical bonding may occur at points of contact between adjacent metal wires within the fiber metal pad.

According to another exemplary example of the present disclosure, the weld current may be delivered to proximal femoral component 10 in discrete but rapid pulses. The weld current may be delivered to proximal femoral component 10 with as few as 4, 6, or 8 pulses and as many as 10, 12, or 14 pulses, or any value therebetween, for example. Each pulse may be as short as 20 milliseconds, 40 milliseconds, or 60 milliseconds, and as long as 80 milliseconds, 100 milliseconds, or 120 milliseconds, or any value therebetween, for example. Between each pulse, the absence of a weld current may promote bulk cooling of porous layer 22 and substrate 20 without eliminating localized, interfacial heating of porous layer 22 and substrate 20. The cooling time between each pulse may be less than 1 second, and more specifically may be as short as 20 milliseconds, 40 milliseconds, or 60 milliseconds, and as long as 80 milliseconds, 100 milliseconds, or 120 milliseconds, or any value therebetween, for example.

As discussed above, the weld pressure used to hold porous layer 22 against substrate 20 during the resistance welding process should be sufficiently low to avoid deforming porous layer 22. Due to softening and/or melting of substrate 20 along the interface, porous layer 22 may shift or translate slightly toward the softened substrate 20 and may become embedded within the softened substrate 20. Therefore, the total thickness of proximal femoral component 10 (i.e., the combined thickness of porous layer 22 and substrate 20) may decrease during the resistance welding process. For example, the total thickness of proximal femoral component 10 may decrease by approximately 0.1%, 0.2%, 0.3%, or more, during the resistance welding process. However, the thickness of porous layer 22 itself should not substantially change. In other words, any measurable change in thickness of proximal femoral component 10 should result from porous layer 22 shifting into the softened substrate 20, not from the compaction or deformation of porous layer 22 itself. When porous layer 22 is in the form of a fiber metal pad, porous layer 22 may undergo some deformation (e.g., shrinkage) due to the formation of metallurgical bonds within porous layer 22. However, this deformation should not be attributed to the weld pressure.

After delivering current to proximal femoral component 10, substrate 20 and porous layer 22 will begin to cool. During this time, controller 160 may be operated to maintain a forge pressure on the components. The forge pressure (i.e. the pressure at which fixtures 120 a, 120 b, and/or weld heads 130 a, 130 b, hold proximal femoral component 10 after the weld current ceases) may be less than the above-described weld pressure. For example, the forge pressure may be as low as 40 psi (0.3 MPa), 60 psi (0.4 MPa), or 80 psi (0.6 MPa) and as high as 100 psi (0.7 MPa), 120 psi (0.8 MPa), or 140 psi (1.0 MPa), or within any range defined between any pair of the foregoing values. The forge time may be as short as 1 second, 2 seconds, or 3 seconds, and as long as 4 seconds, 5 seconds, or more, for example.

In total, the time required to resistance weld porous layer 22 to substrate 20 using apparatus 100 may be as short as 1 second, 10 seconds, 20 seconds, or 30 seconds, and as long as 1 minute, 2 minutes, 3 minutes, or more, for example. The time required may vary depending on the thickness of porous layer 22, the current generated by current generator 150, and other parameters.

Finally, controller 160 may be operated to return fixtures 120 a, 120 b, and/or weld heads 130 a, 130 b, to their respective open positions (FIG. 4A). Proximal femoral component 10 may then be removed from housing 110 of apparatus 100 with porous layer 22 fixedly secured to substrate 20.

Advantageously, by resistance welding porous layer 22 onto substrate 20, a strong metallurgical bond may be achieved between porous layer 22 and substrate 20. In certain examples, the bond strength between porous layer 22 and substrate 20 may be at least 2,900 psi (20.0 MPa), which is the FDA-recommended bond strength for orthopedic implants. Also, because resistance welding involves localized, interfacial heating of porous layer 22 and substrate 20 and requires short cycle times, degradation of porous layer 22 and substrate 20 may be avoided. As a result, the fatigue strength of substrate 20 and porous layer 22 may be substantially unchanged during the resistance welding process.

Although porous layer 22 is described and depicted herein as being bonded directly to substrate 20 of proximal femoral component 10, it is also within the scope of the present disclosure that porous layer 22 may be pre-bonded to an intermediate layer (not shown), which is then subsequently bonded to substrate 20. A suitable intermediate layer may include, for example, a titanium foil. Both the pre-bonding step between porous layer 22 and the intermediate layer and the subsequent bonding step between the intermediate layer and substrate 20 may involve resistance welding, as described above with reference to FIGS. 3, 4A, and 4B. However, it is also within the scope of the present disclosure that the subsequent bonding step between the intermediate layer and substrate 20 may involve traditional, diffusion bonding.

Examples 1. Example #1—Analysis of Trabecular Metal™ Surface Finish and Thickness

A series of samples were prepared, each sample having a disc-shaped porous component produced using Trabecular Metal™ technology and a disc-shaped Tivanium® substrate. The substrates were substantially the same, but the porous components differed in two aspects—surface finish and thickness—as set forth in Table 1 below.

TABLE 1 Interfacing Surface Finish Porous Component Group of Porous Component Thickness (inches) 1 A Electro discharge machined (EDM) 0.060 B Electro discharge machined (EDM) 0.125 C Electro discharge machined (EDM) 0.250 2 A Net shape 0.060 B Net shape 0.125 C Net shape 0.250 3 A Smeared 0.060 B Smeared 0.125 C Smeared 0.250

Before placing the interfacing surface of each porous component against its corresponding substrate, the interfacing surface of each porous component was treated as set forth in Table 1 above.

In Group 1, the interfacing surface of each porous component was subjected to electro discharge machining (EDM), which broke off some protruding ligaments of the porous component and leveled the interfacing surface, making more ligaments available at the interfacing surface to contact the underlying substrate. Therefore, EDM moderately increased the net contact area of the porous components in Group 1.

In Group 2, each porous component was provided in a net shape and the interfacing surface of each porous component was not subjected to machining after manufacturing, so the bulk porosity of the porous component was retained at the interfacing surface. More specifically, the net-shaped interfacing surface was produced by coating an outer surface of a porous structure (e.g., a reticulated vitreous carbon foam structure) with metal and then maintaining the outer, coated surface without machining or shaping the outer surface. Therefore, the net contact area of the porous components in Group 2 was retained.

In Group 3, the interfacing surface of each porous component was subjected to physical machining to break off some ligaments of the porous component and spread out or “smear” other ligaments of the porous component, which caused a substantial reduction in the surface porosity of the interfacing surface. Therefore, smearing increased the net contact area of the porous components in Group 3.

As a result of these surface treatments, the porous components of Group 2 had the least surface contact with the underlying substrate, while the porous components of Group 3 had the most surface contact with the underlying substrate.

The samples were then assembled by resistance welding. A first quantity of power was applied to weld the 0.060 inch (1.5 mm) thick and the 0.125 inch (3.2 mm) thick porous components (Groups 1A, 1B, 2A, 2B, 3A, and 3B) to their corresponding substrates. A second quantity of power 50% greater than the first quantity of power was applied to weld the 0.250 inch (6.4 mm) thick porous components (Groups 1C, 2C, and 3C) to their corresponding substrates. The average bond strength for the samples of each Group 1-3 is depicted graphically in FIG. 5.

As shown in FIG. 5, the samples of Group 2 had higher average bond strengths than the samples of Groups 1 or 3. Because the porous components of Groups 1 and 3 had more surface contact with the underlying substrates than the samples of Group 2, the inventors suspect that the applied current and heat dissipated across the greater surface contact area, resulting in weaker bonds for the samples of Groups 1 and 3 than the samples of Group 2. In contrast, because the porous components of Group 2 had less surface contact with the underlying substrates than the samples of Groups 1 and 3, the inventors suspect that the applied current and heat was localized at each individual ligament, resulting in stronger bonds for the samples of Group 2 than the samples of Groups 1 and 3.

Also, within each Group 1-3, the samples of subgroups A and C had higher average bond strengths than the samples of the corresponding subgroup B. For example, the samples of Groups 2A and 2C had higher average bond strengths than the samples of Group 2B.

The decrease in bond strength from subgroup A to B within each Group 1-3 may be attributed to the increased thickness of the porous components from 0.060 inch to 0.125 inch. Because the thermal conductivity of tantalum in each porous component (about 54 W/m/K) is greater than the thermal conductivity of titanium in each substrate (about 7 W/m/K), the thicker porous components of each subgroup B may act as heat sinks, conducting the heat generated at the interface away from the interface and into the volume of the porous component.

The increase in bond strength from subgroup B to C within each Group 1-3 may be attributed to the 50% increase between the first quantity of power used to resistance weld the 0.060 inch thick and the 0.125 inch thick porous components and the second quantity of power used to resistance weld the 0.250 inch thick porous components. The increased power produces increased current flow, which results in greater heating and a stronger bond.

2. Example #2—Analysis of Trabecular Metal™ Thickness, Weld Power, and Number of Weld Cycles

Another series of samples were prepared, each sample having a disc-shaped porous component produced using Trabecular Metal™ technology and a disc-shaped Tivanium® substrate. Because the net-shaped porous components (Group 2) achieved the highest bond strengths in Example #1, the porous components of Example #2 were also provided in a net shape. The substrates were substantially the same, but the porous components differed in thickness. Also, the resistance welding process differed in two aspects—power and number of weld cycles—as set forth in Table 2 below.

TABLE 2 Porous Component Weld Power Number of Group Thickness (inches) (kJ) Weld Cycles 4 A 0.060 6 1 B 0.060 6 2 5 A 0.125 6 1 B 0.125 6 2 6 A 0.125 9 1 B 0.125 9 2

The average bond strength for the samples of each Group 4-6 is depicted graphically in FIG. 6. The samples of Groups 4 and 6 had higher average bond strengths than the samples of Group 5. In fact, the samples of Groups 4 and 6 had average bond strengths above 4,000 psi (27.6 MPa), which exceeds the FDA-recommended bond strength of 2,900 psi (20.0 MPa).

About 90.6% of the variation in bond strength may be attributed to the varied thickness of the porous components and the varied weld power. The number of weld cycles was found to be statistically insignificant.

3. Example #3—Analysis of Trabecular Metal™ Thickness and Weld Time

Another series of circular samples were prepared, each sample having a disc-shaped porous component produced using Trabecular Metal™ technology and a disc-shaped Tivanium® substrate. The contact area between each porous component and its underlying substrate was about 5 square inches (32.3 cm²). The substrates were substantially the same, but the porous components differed in thickness. Also, the resistance welding cycle time differed, as set forth in Table 3 below.

TABLE 3 Porous Component Weld Time Group Thickness (inches) (milliseconds) 7 A 0.060 150 B 0.060 200 C 0.060 250 8 A 0.125 150 B 0.125 200 C 0.125 250

After resistance welding the samples, two 1.2 inch (3.0 cm) diameter test coupons were cut from each sample for tensile testing. The bond strength for each test coupon of Groups 7 and 8 is depicted graphically in FIG. 7. As shown in FIG. 7, one of the two test coupons of Group 8B had a bond strength greater than the FDA-recommended bond strength of 2,900 psi (20.0 MPa). However, the other test coupon of Group 8B had a bond strength less than 1,000 psi (6.9 MPa).

The variation in bond strength between corresponding test coupons may be due to non-uniform pressure and/or current across each sample. Physical examination of the remnant material left behind after cutting the 1.2 inch (3.0 cm) diameter test coupons supported the finding of varying levels of bond strength within each sample.

4. Example #4—Comparison Between Resistance Welding and Diffusion Bonding

In addition to tensile testing, metallography was also performed to compare the bond achieved with resistance welding to the bond achieved with diffusion bonding.

When a porous component is diffusion bonded to an underlying substrate, atoms from the porous component and atoms from the substrate inter-diffuse. For example, when a porous component produced using Trabecular Metal™ technology is diffusion bonded to a Tivanium® substrate, tantalum from the porous component diffuses into the substrate, and titanium from the substrate diffuses into the porous component. The diffusion of tantalum into the substrate is shown graphically in FIG. 8, and the diffusion of titanium into the porous component is shown graphically in FIG. 9. The inter-diffusion of tantalum and titanium creates a concentration gradient or an inter-diffusion layer along the interface between the porous component and the substrate. The inter-diffusion layer between the porous component and the substrate is also shown visually in FIG. 10, which is a scanning electron microscope image taken along the interface between the porous component and the substrate of a diffusion bonded sample.

When a porous component is resistance welded to an underlying substrate, little or no inter-diffusion occurs. For example, the tantalum concentration in the porous component remains substantially constant in FIG. 8, and the titanium concentration in the substrate remains substantially constant in FIG. 9. The lack of any significant inter-diffusion layer between the porous component and the substrate is also shown visually in FIG. 11, which is a scanning electron microscope image taken along the interface between the porous component and the substrate of a resistance welded sample.

5. Example #5—Analysis of Weld Pressure

A series of 1 inch (2.5 cm) diameter, disc-shaped samples were prepared, the electrode interface of each sample having a surface area of about 0.79 square inches (5.1 cm²). Each sample had a 0.055 inch (1.4 mm) thick porous component produced using Trabecular Metal™ technology and a 0.130 inch (3.3 mm) thick Tivanium® substrate. The weld pressure was calculated to be 4,160 psi (28.7 MPa), which was comparable to the compressive yield strength of the porous components. As a result of this high weld pressure, the porous components were partially crushed during welding and decreased in average thickness by about 0.022 inch (0.6 mm) or 40% (from 0.055 inch (1.4 mm) to 0.033 inch (0.8 mm)).

6. Example #6—Analysis of Pulse Welding

Another series of 1 inch (2.5 cm) diameter, disc-shaped samples were prepared, each sample having a 0.055 inch (1.4 mm) thick porous component produced using Trabecular Metal™ technology and a 0.130 inch (3.3 mm) thick Tivanium® substrate. Each porous component included an EDM-shaped surface interfacing with the substrate. The resistance welding parameters of Example #6 are set forth in Table 4 below.

TABLE 4 Weld Parameter Setting Approach Pressure 20 psi Approach Time 3 seconds Weld Pressure 800 psi Forge Pressure 45 psi Forge Time 3 seconds Current 24 kA Current Density 30 kA/in² Controlled Atmosphere argon dew point < −60° C. oxygen concentration < 10 ppm

As set forth in Table 5 below, each sample received a different number of weld current pulses, with each pulse lasting 80 milliseconds and the cooling time between each pulse lasting 80 milliseconds. Samples 1-5 were prepared to evaluate up to 10 pulses. Samples 6-11 were prepared to more specifically evaluate between 5 and 10 pulses.

TABLE 5 Sample Number of Pulses 1 1 2 4 3 6 4 8 5 10 6 5 7 6 8 7 9 8 10 9 11 10

A new resistance welding apparatus was designed and built to deliver these weld current pulses in a controlled environment. The apparatus included the AX5000 Atmospheric Enclosure with the BMI-500 Single-Column Gas Purification System, the KN-II Projection Weld Head with cooled, copper alloy electrodes, the IT-1400-3 Transformer, and the ISA-2000CR Inverter Power Supply, all of which are available from Miyachi Unitek Corporation of Monrovia, Calif.

The overall thickness of each sample remained substantially the same before and after welding, indicating that the lower 800 psi (5.5 MPa) weld pressure of Example #5 successfully eliminated the distortion and crushing of the porous component seen in Example #4 above.

Samples 1-5 were subjected to tensile testing. The tensile strength of the weld increased from 0 psi (0 MPa) for Sample 1 (1 pulse) to 6,882 psi (47.4 MPa) for Sample 3 (6 pulses). The tensile strength of the weld remained approximately the same for Sample 4 (8 pulses) and Sample 5 (10 pulses).

Samples 6-11 were then subjected to tensile testing and regression analysis, the results of which are presented graphically in FIG. 12. As shown in FIG. 12, the bond strength increased with each additional pulse. The lower 95% prediction interval intersects the 2,900 psi (20.0 MPa) reference line above 9 weld pulses (see circled intersection point in FIG. 12). Thus, at the given weld parameters, at least 10 weld pulses would be required to consistently produce bond strengths of at least 2,900 psi.

Visual inspection of the samples revealed the formation of noticeable heat-affected zones along both the bond interface (i.e., the interface between the porous component and the substrate) and the electrode interface (i.e., the interface between the sample and the resistance welding electrode). Due to the heat generated during the resistance welding process, the samples may experience discoloration, bleed-through, and/or microstructure changes in such heat-affected zones. FIG. 13, for example, depicts the heat-affected zone that formed along the bond interface of Sample 1 above. It would be possible and within the scope of the present disclosure to machine away or otherwise remove the electrode interface after the resistance welding process. However, it would not be possible to remove the bond interface after the resistance welding process without destroying the bond.

7. Example #7—Analysis of Pulsed Weld Current to Reduce Heat-Affected Zones for Net-Shaped Porous Components

To eliminate the heat-affected zones seen in Example #6 above, another series of 1 inch (2.5 cm) diameter, disc-shaped samples were prepared, each sample having a 0.055 inch (1.4 mm) thick porous component produced using Trabecular Metal™ technology and a 0.130 inch (3.3 mm) thick Tivanium® substrate. Unlike Example #6, each porous component included a net-shaped, not EDM-shaped, interfacing with the substrate. Also, compared to Example #6, the samples were subjected to shorter weld pulses, longer cooling times between pulses, higher weld pressures, and higher weld currents during resistance welding. The resistance welding parameters of Example #7 are set forth in Table 6 below.

TABLE 6 Weld Parameter Setting Approach Pressure 20 psi Approach Time 3 seconds Weld Pressure 1,000 psi Forge Pressure 62 psi Forge Time 3 seconds Number of Pulses 10 Weld Time per Pulse 15 milliseconds Cooling Time 250 milliseconds Controlled Atmosphere argon dew point < −60° C. oxygen concentration < 10 ppm

As set forth in Table 7 below, the samples received pulsed weld currents between 35 kA and 51 kA.

TABLE 7 Weld Current Actual Weld Current (kA) Sample Setting (kA) (for Pulse 1) (for Pulses 2-10) 1a 35 33.9 34.3 1b 35 33.9 34.3 2a 39 37.5 38.0 2b 39 37.6 38.0 3a 43 41.0 41.7 3b 43 41.2 41.6 4a 47 43.7 45.1 4b 47 44.3 45.2 5a 51 46.9 48.5 5b 51 46.9 48.5

As an initial matter, visual inspection of the samples showed that the depth and extent of the heat-affected zones along the bond interfaces were significantly reduced compared to Example #6 or, in some cases, were eliminated altogether. For example, Sample 1 of Example #6 (FIG. 13) has a noticeably larger heat-affected zone than Sample 3b of Example #7 (FIG. 14). Some heat-affected zones remained along the electrode interfaces, but as discussed above, it would be possible to machine away or otherwise remove these electrode interfaces after the resistance welding process.

The samples were also subjected to tensile testing and regression analysis, the results of which are presented graphically in FIG. 15. As shown in FIG. 15, the bond strength increased as the weld current per pulse increased. The lower 95% prediction interval intersects the 2,900 psi (20.0 MPa) reference line at about 43 kA per pulse (see circled intersection point in FIG. 13). Thus, at the given weld parameters, a weld current of at least 43 kA per pulse (or a weld density of at least 54 kA/in² (8.4 kA/cm²) per pulse) would be required to consistently produce bond strengths of at least 2,900 psi.

Additional samples were welded at 46 kA per pulse (or a weld density of about 58 kA/in² (9.0 kA/cm²) per pulse) to confirm this result, but the bond strengths were inconsistent and ranged from 2,387 psi (16.5 MPa) to 4,246 psi (29.3 MPa). Also, visual inspection of these additional samples revealed noticeable heat-affected zones along the bond interfaces. The inventors attribute these inconsistent results, at least partially, to electrode wear and metal transfer onto the electrode.

8. Example #8—Analysis of Pulsed Weld Current to Reduce Heat-Affected Zones for EDM-Shaped Porous Components

Example #7 was repeated with porous components having EDM-shaped (not net-shaped) surfaces interfacing with the substrate. None of the samples reached a bond strength of 2,900 psi (20.0 MPa). Also, the samples formed noticeable heat-affected zones along the bond interfaces.

9. Example #9—Analysis of Weld Pressure and Pulsed Weld Current to Reduce Electrode Damage and Improve Bond Strength

To improve the results of Example #7, including reducing general electrode wear and metal transfer onto the electrode, Example #7 was repeated at a higher approach pressure of 40 psi (0.3 MPa), a higher weld pressure of 2,000 psi (13.8 MPa), and a lower forge pressure of 55 psi (0.4 MPa). In accordance with Example #7, the samples were subjected to pulsed weld currents greater than 43 kA, specifically between 45 kA and 61 kA.

At the higher weld pressure of Example #9 (2,000 psi), the inventors noticed less sticking between the samples and the electrodes than at the lower weld pressure of Example #7 (1,000 psi). The present inventors believe that the higher weld pressures increased contact between the samples and the electrodes, and therefore reduced the resistance and the heat generated between the samples and the electrodes.

The samples were subjected to tensile testing and regression analysis, which indicated that a weld current of at least 59 kA per pulse (or a weld density of at least 75 kA/in² (11.6 kA/cm²) per pulse) would be required to consistently produce bond strengths of at least 2,900 psi (20.0 MPa).

Nine additional samples were welded at about 59 kA per pulse to confirm this result, and the bond strengths of these nine additional samples averaged 4,932 psi (34.0 MPa), ranging from 3,174 psi (21.9 MPa) to 6,688 psi (46.1 MPa). Also, visual inspection of these nine additional samples showed minimal presence of heat-affected zones along the bond interfaces.

Six additional samples were welded at about 61 kA per pulse (or a weld density of about 77 kA/in² (11.9 kA/cm²) per pulse) to further confirm this result. Each of these additional samples had a slighter thicker substrate, specifically a 0.170 inch (4.3 mm) thick substrate, compared to previous tests, where the substrates were 0.130 inch thick (3.3 mm). The bond strengths of these additional samples averaged 3,968 psi (27.4 MPa), ranging from 3,259 psi (22.5 MPa) to 4,503 psi (31.0 MPa). In all of these additional samples, tensile failure occurred within the porous material, not along the bond interface between the porous material and the substrate. Also, visual inspection of these samples showed no macroscopic heat-affected zones along the bond interfaces. Furthermore, visual inspection of these samples showed microstructure changes along the electrode interfaces, but at very shallow, removable depths (e.g., less than 0.020 inch (0.5 mm) in depth).

While this invention has been described as having exemplary designs, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims. 

What is claimed is:
 1. A method of manufacturing an orthopedic prosthesis comprising the steps of: providing the orthopedic prosthesis having an outer porous metal layer and an inner metal substrate with the porous metal layer positioned against the metal substrate at an interface therebetween; providing an electrode; providing an intermediate element comprising a distinct member that is separable from the electrode and the orthopedic prosthesis, wherein the intermediate element is positioned between and in contact with the electrode and the porous metal layer, wherein the intermediate element includes tantalum in contact with tantalum of the porous metal layer, and wherein the intermediate element has a surface configured to interface with and match a contour of a surface of the porous metal layer; directing an electrical current to the interface between the porous metal layer and the metal substrate to bond the porous metal layer to the metal substrate; and removing the intermediate element from contact with the porous metal layer after the directing the electrical current to the interface between the porous metal layer and the metal substrate.
 2. The method of claim 1, wherein the intermediate element includes a coating layer that is formed onto the electrode.
 3. The method of claim 1, wherein the intermediate element is a coating of pure tantalum that is applied to the electrode.
 4. The method of claim 1, wherein the surface of the intermediate element comprises a textured outer surface.
 5. The method of claim 1, wherein the intermediate element is affixed to said electrode.
 6. The method of claim 1, wherein the intermediate element includes a copper-containing substrate with a tantalum-containing layer adhered to the copper-containing substrate.
 7. The method of claim 6, wherein the tantalum-containing layer is formed onto the copper-containing substrate.
 8. A method of manufacturing an orthopedic prosthesis comprising the steps of: providing the orthopedic prosthesis having an outer porous metal layer and an inner titanium-containing substrate with the porous metal layer positioned against the titanium-containing substrate at an interface therebetween; providing an electrode; providing an intermediate member positioned between and in contact with the electrode and the porous metal layer, wherein the intermediate member comprises a distinct member from the electrode and the orthopedic prosthesis and is formed separately from the electrode and the orthopedic prosthesis, wherein the intermediate member includes tantalum in contact with tantalum of the porous metal layer, and wherein the intermediate member has a surface configured to interface with and match a contour of an outer surface of the porous metal layer; and directing an electrical current to the interface between the porous metal layer and the titanium-containing substrate to bond the porous metal layer to the titanium-containing substrate.
 9. The method of claim 8, wherein the intermediate member is affixed to the electrode.
 10. The method of claim 8, wherein the intermediate member includes a copper-containing substrate.
 11. The method of claim 8, wherein the intermediate member includes a textured outer surface that comprises the surface in contact with the porous metal layer.
 12. The method of claim 8, wherein the surface of the intermediate layer contacting the porous metal layer is coated so as to comprise a coated surface.
 13. The method of claim 12, wherein the coated surface includes a metallic coating.
 14. The method of claim 8, wherein the porous metal layer includes a net surface, formation of the net surface including: providing a porous structure including the outer surface; coating the outer surface of the porous structure with metal to produce a coated outer surface; and maintaining the coated outer surface without machining the coated outer surface to arrive at the net surface.
 15. A method of manufacturing an orthopedic prosthesis comprising the steps of: providing the orthopedic prosthesis having an outer porous metal layer and an inner metal substrate with the porous metal layer positioned against the metal substrate at an interface therebetween; providing an electrode; providing an intermediate element positioned between and in contact with the electrode and the porous metal layer, wherein the intermediate element comprises a distinct member from the electrode and the orthopedic prosthesis and is formed separately from the electrode and the orthopedic prosthesis, wherein the intermediate member includes tantalum in contact with tantalum of the porous metal layer and includes a copper-containing substrate, and wherein the tantalum of the intermediate element comprises a coating adhered over the copper-containing substrate to limit transfer of the copper-containing substrate to the porous metal layer, and wherein the intermediate element is configured to provide a uniform pressure distribution to the porous metal layer, preserves a porous outer surface of the porous metal layer, and controls material transfer to the metal substrate from one of the intermediate member or electrode; and directing an electrical current to the interface between the porous metal layer and the metal substrate to bond the porous metal layer to the metal substrate.
 16. The method of claim 15, wherein the intermediate element comprises a sheet or layer that is formed separately from the electrode.
 17. The method of claim 16, wherein the intermediate element includes a textured outer surface in contact with the porous outer surface of the porous metal layer.
 18. The method of claim 15, wherein the intermediate element is affixed to the electrode. 